The present invention relates to a process and apparatus for forming a sheet element with a contoured surface from a plane titanium or titanium alloy sheet by hot deep drawing in a press.
In certain cases, it is necessary to fabricate contoured-surface shapes, particularly cylindrical or frustroconical, for example, for fabricating tubes whose thickness may reach or exceed 10 mm, from titanium or titanium alloy, particularly the alloy containing 6 wt. % aluminum and 4 wt. % vanadium, generally known as TA 6V, which has good properties in terms of lightness (density 4.5), mechanical strength (tensile strength at least 900 MPa) and corrosion resistance, particularly to seawater. Such tubes or tube elements would be particularly suitable for connecting a drilling platform and drilling head to the deep sea bed, since steel tubes or tube elements would be too heavy. The alloy containing 3 wt. % aluminum and 2 wt. % vanadium, known as TA 3V, could also be suitable for such applications.
Forming such tubes from plane sheets first means forming the plane sheet into a contoured surface element, particularly cylindrical or conical with a center angle (angle formed by the planes passing through the axis and the extreme generatrices) of, for example, 180.degree. or 120.degree., followed by welding the elements obtained along generatrices to form the tube or a closed shell. The prior art confirmed by testing indicates that such forming cannot be done by a cold process since the mechanical properties of the sheet do not lend themselves to this deformation without becoming unusable. In particular, it is impossible to control precisely the dimensional characteristics of the fabricated part. However, hot forming is a relatively complex operation generally carried out by positioning the sheet in a tool which is itself preheated above the minimum temperature at which forming is to take place. The tool must be heated to the desired temperature before each forming operation, which necessitates a relatively slow rate of fabrication.
The purpose of the present invention is to provide a process for fabricating contoured-surface sheet elements, particularly cylindrical or conical, that is simpler and faster and which nonetheless produces elements of regular thickness and precise dimensions with no cracks. The forming process according to the invention is characterized by the plane sheet element being preheated to a temperature of at least 730.degree. C., by the preheated element being transferred to a non-preheated deep-drawing tool placed in the press soon enough for the temperature of the element to be still at least 700.degree. C. at the time of the deep-drawing operation, by transferring the deep-drawn element to a calibration tool and heating the deep drawn element to at least 650.degree. C. for a sufficient time for its definitive shaping, then allowing the so-processed element to cool to room temperature. The process of the present invention is particularly suitable for forming a sheet element 25 mm thick or less, and the deep-drawn element is preferably kept at a temperature of at least 650.degree. C. in the calibration tool for at least one hour.
For forming such a sheet element, according to the present invention, the temperature of the deep-drawn sheet element is brought to about 650.degree. C. in at least 5 hours and held at about 650.degree. C. for approximately one hour, allowed to cool in the calibration tool for at least 10 hours, and then is removed from this tool and allowed to air-cool.
The formed sheet element may, according to the present invention, be descaled by shot-blasting and, for this purpose, steel balls with diameters between 0.6 millimeters and 0.16 millimeters are used for shot-blasting.
After shot-blasting, according to the invention, the element is finished by being pickled in a cold bath of an aqueous solution with 15 to 40 wt. % nitric acid and 1 to 2 wt. % hydrofluoric acid, with the nitric acid/hydrofluoric acid weight ratio being higher than 10 and the solution containing less than 10 g/l of the total (iron+titanium) for 1 to 5 minutes followed by rinsing and drying.
If, after forming, blasting, and the finishing stage of chemical pickling, the element is left in storage for a relatively long period of time, a patina forms which negatively affects its appearance but not its properties. It can then be chemically pickled in a cold bath similarly to the finishing stage of chemical pickling.
The present invention also relates to apparatus for forming a contoured-surface sheet element from a plane sheet, with the apparatus included a combination of means for heating said plane sheet, a conveyor, a press, and calibration means, these various elements being distinct from each other.
The press may, in accordance with the present invention, be fitted with at least one insert, and, may have a furnace designed to receive the calibration means. Additionally, the mechanical descaling means may be provided for carrying out a descaling operation.
An apparatus for forming cylindrical sheet elements and a method for forming and descaling according to the invention is described hereinbelow as an example with reference to the accompanying drawings.